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555 Timer Ball Whacker 

Make] Projects 

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build, hack, tweak, share, discover,- 

555 Timer Ball Whacker 

Written By: Steve Hobley 


Servo Motor (1) 

from an RC plane/boat/car shop 

Breadboard (1) 
from RadioShack 

Resistor (1) 
from RadioShack. 

Photoresistor (1) 
from RadioShack. 

Diodes (1) 

from RadioShack. 

Desk lamp (1) 

Plywood (1) 

Polystyrene ball (1) 

or other lightweight, swattable object. 

Carriage bolt (1) 

Batteries (3) 
from RadioShack. 

Battery Holder 3XAAA (1) 
© Make Projects Page 1 of 6 

555 Timer Ball Whacker 

from RadioShack. 

555 Timer IC (1) 
from RadioShack. 

Electrolytic Capacitor 1.0uF (1) 
from RadioShack 

Electrolytic Capacitor IQOOuF (1) 
from RadioShack 

Jeweler's wire (1) 


This project uses a simple 555 timer chip and a feedback loop to control a servo-controlled 
wooden arm. Whenever an object comes close to a photosensor mounted on the end of the 
arm, it blocks the amount of light detected, which triggers the arm to swat the object away. 

Check out more Weekend Projects . 

© Make Projects Page 2 of 6 

555 Timer Ball Whacker 

Step 1 — Create the Stand 

• I designed a simple frame to hold all the parts, cut from 1/4" birch plywood. 

• For this design, you can use a 4" carriage bolt across the top, from which gold wire and 
thread can be tied to suspend the "ball," in this case, a plastic egg. 

• Here's a simple template that can be used to cut out your own stand. Feel free to tweak it 
to your liking. 

© Make Projects 

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555 Timer Ball Whacker 

Step 2 — Populate the Breadboard 

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• Install the 555 timer on the breadboard, straddling the central "trench." 

• Add the resistors: 1.5kQ (Brown-Green-Red-Gold), 47kQ (Yellow- Violet-Orange-Gold), 
15kQ (Brown-Green-Orange-Gold), 2.2kQ (Red- Red- Red-Gold), 10kQ (Brown-Black- 

• Add the 1000 F and 1 F capacitors. These are polarized and need to go in the 
right way around. 

• Add the diode. This is also polarized and needs to go in the right way around. 

• The schematic here shows all of the connections. 

© Make Projects 

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555 Timer Ball Whacker 

Step 3 — Connect the Off board Components 

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• Wire up the servo's red (+5v), black (Gnd), and yellow (Control) connections from the 
circuit on the breadboard. 

• Connect the photoresistor (aka light-dependent resistor or LDR) to the circuit using wires 
long enough to reach up the wooden arm. 

• Drill a small hole in the end of the arm to hold the photodiode and insert it. Tape the wires 
down the arm to the breadboard. 

• Attach the wooden arm to the servomotor such that it leans slightly away from the ball. 
Mount the breadboard and servomotor onto the stand. 

• Connect the battery holder to the power and ground rails along the sides of the breadboard. 

• The connections you may not be able to see on the breadboard (second photo) are: 
Diode to Pin 6 of the IC, large cap between +5V and the 6th column of holes, small 
cap between Ground and Pin 2 of the IC. 


© Make Projects 

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555 Timer Ball Whacker 

Step 4 — Final Setup 

• I built the frame from what I had lying around. To hold the servo and breadboard, I glued 
down a few off-cuts of plywood. 

• For the arm, I used a 6" piece of square dowel. I wrapped the photoresistor wires around it 
and secured them with tape. 

• I used jeweler's wire to secure the arm to the servo — just loop the wire around and twist it 

• Position a desk lamp so that it shines light towards the photosensor, from the other side of 
the hanging object. 

• Insert batteries into the battery holder and watch the fun. If the arm swings the wrong way, 
turn the servomotor around. 

• It might take some tweaking to find the "sweet" spot" where the light/sensor/ball all align to 
create the whacking action. But when you get it, it's pretty funny to watch... 

• You can see the ball whacker in action here. 

This project is a great example of a cybernetic , or "self-governing" system. These kinds of 
systems have been around since the steam age (check out fly-ball governors for an early 
example of the technology). 

With this kind of analog robotic control system, remarkably human-like behaviour can be 
obtained without the need for complex digital programming. 

This document was last generated on 201 2-1 1 -02 09:47:01 AM. 

© Make Projects 

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